If I buff a 2 might unit to 4 might in response to Gust targeting it, what happens to Gust?

Ruling: Gust still resolves and triggers any relevant effects (like Darius or Student abilities), but it has no effect on the now-illegal target.

Nuances:

• The target of Gust is locked in when the card is finalized and cannot be changed, even if other legal targets exist when it resolves
• The card still "resolves" for trigger purposes even though its effect does nothing